1. Field of the Invention
The invention relates to a spectrophotometer. More particularly, it relates to an improved spectrophotometer especially useful for optical analysis of samples in a centrifugal analyzer.
2. Background Art
Spectrophotometers are well known in the art, and comprise various main components. The light source usually comprises one or two continuous-emission lamps, e.g., halogen, deuterium, mercury arc and/or xenon. Prism or grating devices are used for the dispersion. The detectors are generally photomultipliers, or silicon photodiodes. The numerous kinds of available components and the various possible structures can be combined in numerous ways to construct a spectrophotometer having desired characteristics for a particular intended use.
The disadvantages of known spectrophotometers are mainly due to the light source and the monochromator used. Particularly in spectrophotometers used in centrifugal analyzers, it is desirable to use a light source such as a flash lamp in order to enable high speed absorbance measurements. Accordingly, although the tungsten-halogen lamp is undoubtedly the most frequently-used source for applications in the visible spectrum, it has the following well-known disadvantages:
(a) very weak emission of ultraviolet light;
(b) very great variation of light intensity in the useful spectrum; the intensity of light at 290 nm is about 900 times less than at 700 nm;
(c) the proportion of interfering light is considerable, which means that expensive blocking filters need to be used;
(d) the system for compensating variations in light intensity with wavelength must have a wide dynamic range;
(e) the service life is relatively short;
(f) the light output is relatively low;
(g) the dissipated power is considerable; and
(h) the electric supply means for the lamp is relatively heavy and bulky.
In order to alleviate the disadvantages with the aforementioned kind of lamp, light can be obtained from lamps producing different kinds of electric (flash) discharges, e.g. xenon, mercury or argon lamps. Xenon lamps have the most uniform spectrum, and their light efficiency is much greater than that of tungsten. However, the lamps are usually supplied for higher power than 100 W, and are very difficult to cool. Also, the lamp supply and mounting means are very voluminous and expensive.
The grating monochromator is a preferred method of continuously varying the wavelength. However, the proportion of interfering light resulting from the associated lamp and monochromator must be very low, e.g., less than 1.times.10.sup.-4, if it is desired to make measurements having a low linearity error (e.g. lower than 1.5% up to an attenuation of 1000 times (corresponding to an absorbance of 3)). To obtain performance of this kind, double-grating monochromators are ordinarily used, since single-grating monochromators have too high a proportion of interfering light. However, double-grating mmonochromators are expensive, bulky and take a relatively long time to align.
U.S. Pat. No. 3,810,696 discloses a spectrophotometer comprising a flash tube and an interference filter or a monochromator to produce two light beams, the first of which travels through a sample for analysis and the second of which reaches a detector which delivers a reference signal corresponding to the intensity of the second beam. U.S. Pat. No. 4,241,998 discloses a spectrophotometer intended in particular for the optical analysis of samples in a centrifugal analyzer. The spectrophotometer comprises a flash tube, a stabilizing optical device for deriving a light beam having a constant spatial distribution from each flash from the flash tube, a grating monochromator for dispersing the light delivered by the stabilizing device and for delivering a beam of filtered light, an optical element for dividing the filtered beam to produce two beams, the first of which travels through a sample for analysis and the second of which reaches a detector which delivers a reference signal corresponding to the intensity of the second beam, and a second detector placed to receive the beam emerging from the sample.